This applet can be used to/explore the addition law for mutually exclusive events.[br][br]e.g. Using the default tank, highlight region A and ask what is the probability of fish being in region A, P(A)?[br][br]By considering the areas [math]P\left(A\right)=\frac{10}{48}[/math] [br][br][br]Uncheck and check D. Whats is the probability of a fish being in region D, P(D)?[br][br][math]P\left(D\right)=\frac{12}{48}[/math][br][br]Check A... what is the probability of the fish being in region A or D. Now it should be evident that this is ...[br][br][math]\frac{\text{area of A + area of D }}{\text{area of the tank [br]}}=\frac{10+12}{48}=\frac{24}{48}=\frac{1}{2}[/math][br][br]This is a demonstration of why we should add if we have an or probability problem and the combined probability in a or problem will always increase. The next question to ask is what assumptions have we made?; mutually exclusive events, i.e the fish cant be half-in one region and half in another. Is this realistic in the context of the problem?[br][br]The fish tank can be rescaled and the divisions changed by dragging the cross-hairs to generate other examples.[br][br]There is also a student worksheet for this applet attached. [br]